There is concern that the transport of injured patients by road can have a detrimental effect on their medical condition. To a significant extent, this detrimental effect can be attributed to the transmission of oscillations of the vehicle to the patient. Vertical oscillations of the vehicle are experienced as front to back vibrations of a supine patient, and it is known that humans are sensitive far more to front to back vibrations than to up-and-down vibrations, such as are experienced by a seated or standing person in a vehicle.
There is therefore a need to support supine patients in ambulances in a manner which will isolate the patient from vertical vibrations of the ambulance. Since, however, the natural frequency of any conventional spring-mass system depends on the load, and consequently on the weight of the person being carried, a system giving adequate vibration isolation for one person would not perform satisfactorily for another person of different weight. Furthermore, it is desirable for the mean ride level of the patient to be independent of their weight, in order to assist ambulance staff in performing any necessary treatment on the patient.
GB1447261 discloses a suspension device for a vehicle seat. The device comprises two pairs of levers mounted in a scissor arrangement. The lower ends of the levers of each pair are movable in a frame fixed to the vehicle, while the upper ends of the levers carry the seat. The pivots of the two pairs of levers are interconnected by a shaft which moves along a cam surface as the seat moves up and down. Springs act to bias the shaft towards the upper end of the cam surface, so biasing the seat upwardly. The inclination of the cam surface is adjustable so as to adjust the spring rate of the springs as applied to the seat. Thus, the suspension device can be adapted to the weight of a person sitting in the seat.
The device of GB1447261 is apparently adjusted by the occupant of the seat to obtain a firmer or softer ride according to his own preference. It is likely that the occupant would make repeated adjustments over a period of time until the subjective optimum is reached. However, such a progressive adjustment is not normally possible, nor necessarily desirable, in suspension devices used for supporting sick or injured people in ambulances. In such cases, the need is to provide a suspension system which automatically adjusts itself in dependence on the weight of the patient, in order to avoid, as far as possible, the transmission of vibrations from the vehicle to the patient during travel under emergency conditions.